First of all, I would suspect that the majority of Protestants, if not all on here, do NOT follow Joel Osteen. I cannot stand the guy and his so-called "preaching." I feel as if I need an insulin shot, when I have listened to him on TV. One never hears law or gospel in his "sermons." No Good News. No, just speech to generate the warm fuzzies. So Osteen is nowhere near to Protestants the way the Pope is to Catholics. Osteen has zero authority over us and what we believe.
Not all Protestant churches teach "OSAS," but I fail to see how that has any relation to Purgatory. A concept can be found in the Bible without any label attached to it--like the Trinity, Virgin Birth, etc. But there is zero concept for Purgatory in the Bible. Nada. Zilch. Nothing. It is appointed for us to die once and then the judgment. (Hebrews).